With the miniaturization of electronic equipment, electronic components mounted on printed wiring boards and the like in the electronic equipment have also been miniaturized, and surface-mount type electronic components have been widely used. Examples of the surface-mount type electronic components include various quartz crystal devices in which at least a quartz crystal blank is housed in a container for surface mounting. Such crystal devices include: crystal units in which a crystal blank is hermetically enclosed in a container; and crystal oscillators in which a crystal blank is hermetically enclosed in a container, and an IC chip (integrated circuit chip) in which an electronic circuit, such as an oscillation circuit using the crystal blank, is integrated is housed in the container. The surface-mount type crystal units and crystal oscillators have small size and light weight, so that they are included as frequency and time reference sources, particularly in portable electronic equipment.
In the surface-mount type crystal devices, the crystal blank should be piezoelectrically vibrated in a state cut off from the ambient air, so that a configuration is often used in which using a container made of ceramic, the crystal blank is held and hermetically enclosed in a recess formed in such a container. The crystal device in which the crystal blank is hermetically enclosed in the container made of ceramic in such a manner is surface-mounted on a wiring board by, for example, reflow soldering.
However, with the miniaturization of the surface-mount type crystal devices, the volume of solder in the bonding portion between the crystal device and the wiring board also decreases, and stress due to the difference between the thermal expansion coefficient of the container made of ceramic in the crystal device and the thermal expansion coefficient of the wiring board is concentrated in the solder in the bonding portion, so that cracks may occur in the solder.
FIGS. 1A to 1C show a general surface-mount type crystal unit. FIG. 1A is a cross-sectional view of the crystal unit mounted on a wiring board, FIG. 1B is a bottom view of the crystal unit, and FIG. 1C is a partly enlarged cross-sectional view showing the configuration of a mounting terminal in the crystal unit.
In the crystal unit shown in the figures, crystal blank 2 is housed in surface-mount type container body 1, and metal cover 3 is placed over container body 1 to hermetically enclose crystal blank 2 in the container. Container body 1 has a generally rectangular planar contour, that is, a flat, generally rectangular parallelepiped-shaped contour, which is seen as a rectangle as seen from above when container body 1 is mounted on a wiring board. A recess for housing crystal blank 2 is formed on the upper surface of container body 1. Such a container body is constituted of laminated ceramic so as to have generally rectangular bottom wall layer 1a, and frame wall layer 1b that is formed in a frame shape by providing a generally rectangular opening, and is laminated on bottom wall layer 1a. A pair of crystal holding terminals 4 are provided on the inner bottom surface of the recess in the vicinity of positions at both ends of one side of the inner bottom surface respectively. Crystal holding terminals 4 are used for electrically and mechanically holding crystal blank 2 in the recess, as described later.
External terminal 6 as the mounting terminal used in mounting container body 1 on a wiring board is provided in four corner portions on the outer bottom surface of container body 1, that is, the surface that faces the wiring board when container body 1 is mounted on the wiring board. External terminal 6 is formed as a conductive layer having a generally rectangular planar shape. Part of external terminal 6 is provided as a side electrode, extending to a side surface of container body 1. Of these four external terminals 6, a pair of external terminals 6 positioned at both ends of one diagonal in the outer bottom surface of container body 1 are electrically connected to the pair of crystal holding terminals 4 via conductive paths formed on the side surfaces of container body 1 and the lamination plane in the laminated ceramic. The remaining two external terminals 6 are used as ground terminals.
Crystal blank 2 is made of, for example, a generally rectangular AT-cut quartz crystal blank, and excitation electrodes 7 are formed on both major surfaces, respectively, as shown in FIG. 2. Extraction electrodes 8 are extended from the pair of excitation electrodes 7 toward both sides of one end of crystal blank 2. At a position at an end of crystal blank 2, each extraction electrode 8 is formed to fold back between both major surfaces of crystal blank 2. Crystal blank 2 is fixed and held in recess 2 of container body 1 and is electrically and mechanically connected to container body 1 by fixing these extraction electrodes 8 to crystal holding terminals 4 respectively by, for example, conductive adhesive 9 or the like, at positions to which the pair of extraction electrodes 8 are extended.
A metal ring (not shown) is provided on the upper surface of container body 1 to surround the recess, and metal cover 3 is bonded to this metal ring by seam-welding or the like. The metal ring is electrically connected to external terminals 6 used as the ground terminals via through holes formed in container body 1.
External terminal 6 provided on the outer bottom surface of container body 1 is generally formed by laminating layers of tungsten (W), nickel (Ni), and gold (Au), in order, on the ceramic layer of container body 1, as shown in FIG. 1C. Different types of metals are laminated in this manner to constitute external terminal 6 in order to increase the bonding strength between the ceramic layer on container body 1 side and solder 10 used for bonding to wiring board 5. Here, tungsten is selected in terms of bonding strength to ceramic, nickel is selected to obtain properties required as the electrode body, and gold is selected in terms of adhesive strength to solder 10. The tungsten layer and the gold layer in external terminal 6 can be thinly formed as long as bonding strength or adhesive strength can be obtained.
Therefore, the thickness of external terminal 6 depends on the thickness of the nickel layer as the electrode body. In conventional crystal units, for example, the thickness of the nickel layer is about 0.003 to 0.009 mm (that is, 3 to 9 μm), while the thickness of the tungsten layer is about 1 μm, and the thickness of the gold layer is about 0.5 μm. When external terminal 6 is formed, in laminating ceramic green sheets, that is, unfired raw ceramic sheets, to form container body 1, the tungsten layer is provided on the ceramic green sheet forming container body 1 by a printing method and is finally formed by integrally firing with container body 1. Also, the nickel layer and the gold layer are formed by, for example, electrolytically plating the tungsten layer after firing.
The surface-mount type crystal unit configured in this manner is mounted on wiring board 5 of, for example, a glass-epoxy resin material, by reflow soldering. Specifically, circuit terminals 11 of copper (Cu), corresponding to external terminals 6 of the crystal unit, are previously provided on a surface of wiring board 5, and by bonding external terminals 6 to such circuit terminals 11 by reflow soldering, the crystal unit is surface-mounted on wiring board 5. At this time, solder 10 also climbs on the side portions of container body 1 to form the so-called solder fillets. By checking whether the solder fillets are properly formed, whether solder 10 melts during reflow soldering can be checked. When the solder fillets are not formed, it can be determined that reflow soldering is not suitably performed. Circuit terminals 11 are generally formed by patterning a copper (Cu) layer provided on the surface of circuit board 5.